Electrosurgical instruments are a type of surgical instrument used in many surgical operations. Electrosurgical instruments apply electrical energy to tissue in order to treat tissue. An electrosurgical instrument may comprise an instrument having a distally-mounted end effector comprising one or more electrodes. The end effector can be positioned against tissue such that electrical current is introduced into the tissue. Electrosurgical instruments can be configured for bipolar or monopolar operation. During bipolar operation, current is introduced into and returned from the tissue by active and return electrodes, respectively, of the end effector. During monopolar operation, current is introduced into the tissue by an active (or source) electrode of the end effector and returned through a return electrode (e.g., a grounding pad) separately located on a patient's body. Heat generated by the current flow through the tissue may form hemostatic seals within the tissue and/or between tissues and thus may be particularly useful for sealing blood vessels, for example. The end effector of an electrosurgical instrument sometimes also comprises a cutting member that is moveable relative to the tissue and the electrodes to transect the tissue.
Electrical energy applied by an electrosurgical instrument can be transmitted to the instrument by a generator. The generator may form an electrosurgical signal that is applied to an electrode or electrodes of the electrosurgical instrument. The generator may be external or integral to the electrosurgical instrument. The electrosurgical signal may be in the form of radio frequency (“RF”) energy. For example, RF energy may be provided at a frequency range of between 100 kHz and 1 MHz. During operation, an electrosurgical instrument can transmit RF energy through tissue, which causes ionic agitation, or friction, in effect resistive heating, thereby increasing the temperature of the tissue. Because a sharp boundary may be created between the affected tissue and the surrounding tissue, surgeons can operate with a high level of precision and control, without sacrificing un-targeted adjacent tissue. The low operating temperatures of RF energy may be useful for removing, shrinking, or sculpting soft tissue while simultaneously sealing blood vessels. RF energy may work particularly well on connective tissue, which is primarily comprised of collagen and shrinks when contacted by heat.
Short circuits are a recurrent problem for electrosurgical instruments. For example, if a conductive clip, staple or other non-tissue conductive object is present between the electrodes of an electrosurgical instrument and touching both polarities simultaneously, electrosurgical energy can be shunted through the conductive object. Additionally, in the case of bipolar forceps, the electrodes can touch each other during normal usage. This contact shunts electrical energy away from the tissue and the surgeon has to open the forceps and re-grasp the tissue. This can result in several undesirable outcomes including, for example, incomplete tissue effect, excessive heating of the conductive object, a delay of the surgery, clinician inconvenience or frustration, etc. Existing methods for coping with short circuits utilize the generator or other suitable component to determine when the impedance between the electrodes falls below a threshold value, for example, for a threshold amount of time. When such an impedance drop is detected, the generator alerts the clinician, who can then reposition the electrodes and/or remove the conducting object. Existing methods, however, suffer when tissue impedance itself drops during treatment. For example, during electrosurgical treatment, localized tissue impedance can often fall as low as just a few ohms. Existing methods are often inadequate for distinguishing between short circuits and normally occurring low tissue impedance.